Clinico-pathological. analysis on Babesia Microti and Babesiarodhaini infection in mice were performed.1) Immunological responses of splenic cell from mice inoculated with B. rodhaini were examined. Splenic cells form mice-inoculated wlth B. rodhaini were significantly proliferated against homogenous Babesia soluble lysate antigens and also enhanced Interleukin 2 production. The major proliferative cells were seemed to be T cell, especially helper T cell, because of the proliferative responses were nearly completely abolished by pre-treatments of splenic cells with anti-Thy-1 and Anti Lyt-1.2 antibodies. Splenic cells from immunized mice showed protective activity against B. rodhaini inoculation, whereas splenic"cells from hyperimmunized mice showed. no activity. However, pretreated With anti-T cell, ant-Lyt-1.2, and anti Lyt-2.2 antibodies, splenic cells revealed protective activities against B. rodhaini inoculation. Changes of splenic lymphocyte subpopulation after B. microti and B. rodhaini were examined. The Thy-1 positive cell number in B. microti inoculated mice were significantly higher than that in B. rodhaini inoculated mice. The ratio in L3T4/Lyt-2 positive cell after inoculation with-B. rodhaini inoculated mice revealed a lack of an increasing phase. These results suggested that the T cell dependent early immune response, especially suppressor activity, was closely related to the difference in the course of infection between the non-lethal B. microti and the lethal B. rodhaini infection in mice.2) Isoenzyme patters of glucose-6-phosphate -dehydrogenase (G6PD), malate dehydrogenase (MDH), and lactate dehydrogenase (LDH) were examined. There were no difference in G6PD and MDH between 2 strains, however, the quite difference in LDH isoenzyme pattern were observed between these 2 strains.